Prostheses, Systems and Methods for Replacement of Natural Facet Joints With Artificial Facet Joint Surfaces

ABSTRACT

Cephalad and caudal vertebral facet joint prostheses and methods of use are provided. A pair of fixation elements are adapted to be secured within a vertebra in an orientation that best assures a secure and durable attachment to cortical and/or cancellous bone. Artificial facet joint surfaces are mounted on the fixation elements, either directly or with the aid of a support. The artificial facet joint structure may be carried by an arm. The artificial facet joint structure is adapted for articulation with a complementary natural or artificial facet joint structure. Bilateral prostheses may by coupled by a brace to further secure and stabilize the prostheses.

CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.10/974,009, filed Oct. 25, 2004, and entitled “Prostheses, Systems andMethods for Replacement of Natural Facet Joints with Artificial FacetJoint Surfaces,” which is a continuation-in-part of U.S. patentapplication Ser. No. 10/158,563, filed May 30, 2002, and entitled“Prostheses, Systems and Methods for Replacement of Natural Facet Jointswith Artificial Facet Joint Surfaces,” which is a continuation-in-partof U.S. patent application Ser. No. 10/067,137, filed Feb. 4, 2002, andentitled “Facet Arthroplasty Devices and Methods,” which is acontinuation-in-part of U.S. patent application Ser. No. 09/693,272,filed Oct. 20, 2000, and entitled “Facet Arthroplasty Devices andMethods,” which claims the benefit of Provisional Patent ApplicationSer. No. 60/160,891, filed Oct. 22, 1999, and entitled “FacetArthroplasty Devices and Methods,” all of which are incorporated hereinby reference.

FIELD OF THE INVENTION

This invention relates to prostheses for treating various types ofspinal pathologies, as well as to methods of treating spinalpathologies.

BACKGROUND OF THE INVENTION

I. Vertebral Anatomy

As FIG. 1 shows, the human spinal column 10 is comprised of a series ofthirty-three stacked vertebrae 12 divided into five regions. Thecervical region includes seven vertebrae 12, known as C1-C7. Thethoracic region includes twelve vertebrae 12, known as T1-T12. Thelumbar region contains five vertebrae 12, known as T1-T5. The sacralregion is comprised of five vertebrae 12, known as S1-S5. The coccygealregion contains four vertebrae 12, known as Co1-Co4.

FIG. 2 shows a normal human lumbar vertebra 12. Although the lumbarvertebrae 12 vary somewhat according to location, they share manyfeatures common to most vertebrae 12. Each vertebra 12 includes avertebral body 14. Two short bones, the pedicles 16, extend backwardfrom each side of the vertebral body 14 to form a vertebral arch 18.

At the posterior end of each pedicle 16 the vertebral arch 18 flares outinto broad plates of bone known as the laminae 20. The laminae 20 fusewith each other to form a spinous process 22. The spinous process 22serves for muscle and ligamentous attachment. A smooth transition fromthe pedicles 16 into the laminae 20 is interrupted by the formation of aseries of processes.

Two transverse processes 24 thrust out laterally on each side from thejunction of the pedicle 16 with the lamina 20. The transverse processes24 serve as levers for the attachment of muscles to the vertebrae 12.Four articular processes, two superior 26 and two inferior 28, also risefrom the junctions of the pedicles 16 and the laminae 20. The superiorarticular processes 26 are sharp oval plates of bone rising upward oneach side from the union of the pedicle 16 with the lamina 20. Theinferior processes 28 are oval plates of bone that jut downward on eachside.

The superior and inferior articular processes 26 and 28 each have anatural bony structure known as a facet. The superior articular facet 30faces upward, while the inferior articular facet 31 faces downward. AsFIG. 3 shows, when adjacent vertebrae 12 are aligned, the facets 30 and31, capped with a smooth articular cartilage, interlock to form a facetjoint 32, also known as a zygapopysial joint.

The facet joint 32 is composed of a superior facet and an inferiorfacet. The superior facet is formed by the vertebral level below thejoint 32, and the inferior facet is formed by the vertebral level abovethe joint 32. For example, in the L4-L5 facet joint, the superior facetof the joint is formed by bony structure on the L-5 vertebra (e.g., asuperior articular surface and supporting bone on the L-5 vertebra), andthe inferior facet of the joint is formed by bony structure on the L-4vertebra (e.g., an inferior articular surface and supporting bone on theL-4 vertebra).

As also shown in FIG. 3, an intervertebral disc 34 between each pair ofvertebrae 12 permits gliding movement between vertebrae 12. Thus, thestructure and alignment of the vertebrae 12 permit a range of movementof the vertebrae 12 relative to each other.

II. Facet Joint Dysfunction

Back pain, particularly in the “small of the back”, or lumbosacral(L4-S1) region, is a common ailment. In many cases, the pain severelylimits a person's functional ability and quality of life. Such pain canresult from a variety of spinal pathologies.

Through disease or injury, the laminae, spinous process, articularprocesses, or facets of one or more vertebral bodies can become damaged,such that the vertebrae no longer articulate or properly align with eachother. This can result in an undesired anatomy, loss of mobility, andpain or discomfort.

For example, the vertebral facet joints can be damaged by eithertraumatic injury or by various disease processes. These diseaseprocesses include osteoarthritis, ankylosing spondylolysis, anddegenerative spondylolisthesis. The damage to the facet joints oftenresults in pressure on nerves, also called a “pinched” nerve, or nervecompression or impingement. The result is pain, misaligned anatomy, anda corresponding loss of mobility. Pressure on nerves can also occurwithout facet joint pathology, e.g., a herniated disc.

One type of conventional treatment of facet joint pathology is spinalstabilization, also known as intervertebral stabilization.Intervertebral stabilization prevents relative motion between thevertebrae. By preventing movement, pain can be reduced. Stabilizationcan be accomplished by various methods.

One method of stabilization is spinal fusion. Another method ofstabilization is fixation of any number of vertebrae to stabilize andprevent movement of the vertebrae.

Another type of conventional treatment is decompressive laminectomy.This procedure involves excision of the laminae and/or soft tissues ofthe spine to relieve compression of nerves.

These traditional treatments are subject to a variety of limitations andvarying success rates. Furthermore, none of the described treatmentsputs the spine in proper alignment or return the spine to a desiredanatomy. In addition, stabilization techniques, by holding the vertebraein a fixed position, permanently limit a person's mobility.

SUMMARY OF THE INVENTION

There is a need for prostheses, systems, and methods that overcome theproblems and disadvantages associated with current strategies anddesigns in various treatments for spine pathologies.

The invention provides prostheses, systems, and methods designed toreplace natural facet joints and/or part of the lamina at virtually allspinal levels including L1-L2, L2-L3, L3-L4, L4-L5, L5-S1, T11-T12, andT12-L1. The prostheses, systems, and methods can restore a desiredanatomy to a spine and give back to an individual a desired range ofmobility. The prostheses, systems, and methods also can lessen oralleviate spinal pain by relieving the source nerve compression orimpingement.

For the sake of description, the prostheses that embody features of theinvention will be called either “cephalad” or “caudal” with relation tothe portion of a given natural facet joint they replace. As previouslydescribed, a given natural facet joint has a superior facet and aninferior facet. In anatomical terms, the superior facet of the joint isformed by the vertebral level below the joint (which can thus be calledthe caudal portion of the facet joint, i.e., because it is near thefeet). The inferior facet of the joint is formed by the vertebral levelabove the joint (which can thus be called the cephalad portion of thefacet joint, i.e., because it is near the head). Thus, a prosthesisthat, in use, replaces the caudal portion of a facet joint (i.e., thesuperior facet) will be called a “caudal” prosthesis. Likewise, aprosthesis that, in use, replaces the cephalad portion of a facet joint(i.e., the inferior facet) will be called a “cephalad” prosthesis.

One aspect of the invention provides a facet joint prosthesis toreplace, on a vertebral body, a caudal portion of a natural facet joint(e.g., a superior articular surface and supporting bone structure on thevertebral body). A pair of fixation elements are adapted to be securedwithin the vertebral body in an orientation that best assures a secureand durable attachment to cortical and/or cancellous bone. Artificialfacet joint structures mounted on the fixation elements. In oneembodiment, the artificial facet joint structure is mounted on thefixation element by use of a support. The artificial facet jointstructures articulate with a complementary natural or artificial facetjoint structure. The artificial facet joint structures may by coupled bya brace to further secure and stabilize the prosthesis.

This aspect of the invention also provides a method of replacing, on avertebral body, a caudal portion of a natural facet joint. The methodremoves a caudal portion of the natural facet joint from the vertebralbody. Right and left fixation elements are secured within the vertebralbody, e.g., to right and left pedicles respectively. An artificial facetjoint structure is mounted on each fixation elements. A brace may becoupled to each of the artificial facet joint structures to stabilizethe prosthesis.

Another aspect of the invention provides a facet joint prosthesis toreplace, on a vertebral body, a cephalad portion of a natural facetjoint (e.g., an inferior articular surface and supporting bone structureon the vertebral body). A pair of fixation elements are adapted to besecured within the vertebral body in an orientation that best assures asecure and durable attachment to cortical and/or cancellous bone. In apreferred embodiment, arms are adapted to be mounted on the fixationelements (e.g., using a brace and/or support). The arms carry anartificial facet joint structure for articulation with a complementarynatural or artificial facet joint structure. The arms may by coupled bya brace to further secure and stabilize the prosthesis.

This aspect of the invention also provides a method of replacing, on avertebral body, a cephalad portion of a natural facet joint. The methodremoves a cephalad portion of the natural facet joint from the vertebralbody. In one embodiment, right and left fixation elements are securedwithin the vertebral body, e.g., to the right and left pediclesrespectively. A support is mounted on each fixation element. A bracecarrying right and left arms (carrying the artificial facet jointstructures) is coupled to the supports.

In an alternative embodiment, right and left fixation elements aresecured within the vertebral body, e.g., to the right and left pediclesrespectively. A support is mounted on each fixation element. An arm,carrying an artificial facet joint structure, is mounted on eachsupport. A brace may be coupled to each of the arms to stabilize theprosthesis.

Another aspect of the invention provides a facet joint prosthesis toreplace, on a vertebral body, a caudal portion of a natural facet joint,including: a support component adapted to span a portion of thevertebral body and to support prosthetic caudal facet elements; and apair of prosthetic caudal facet elements adjustable relative to thesupport component and adapted to replace the caudal portion of thenatural facet joint.

Yet another aspect of the invention provides a prosthesis for replacinga natural spinal facet joint including: a pair of prosthetic caudalfacet elements configured to replace the caudal portion of the naturalfacet joint; and a modular cephalad prosthesis configured to articulatewith the caudal facet elements, the modular cephalad prosthesiscomprising a pair of arms, a pair of supports, and a brace extendingbetween the arms and the supports, each of the supports having anarticulating portion adapted to articulate with the caudal facetelements.

Still another aspect of the invention provides a prosthesis forreplacing a natural spinal facet joint comprising: a modular caudalprosthesis comprising a pair of prosthetic caudal facet elementsconfigured to replace the caudal portion of the natural facet joint anda caudal brace extending between the caudal facet elements; and amodular cephalad prosthesis configured to articulate with the caudalfacet elements, the modular cephalad prosthesis comprising a pair ofarms, a pair of supports, and a cephalad brace extending between thearms and the supports, each of the supports having an articulatingportion adapted to articulate with the caudal facet elements.

Other features and advantages of the inventions are set forth in thefollowing Description and Drawings, as well as in the appended Claims.

INCORPORATION BY REFERENCE

All publications and patent applications mentioned in this specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a lateral elevation view of a normal human spinal column.

FIG. 2 is a superior view of a normal human lumbar vertebra.

FIG. 3 is a lateral elevation view of a vertebral lumbar facet joint.

FIG. 4 is an exploded perspective view of cephalad and caudal prosthesesfor replacing, respectively, the inferior and superior halves of anatural facet joint.

FIG. 5 is an assembled perspective view of the prostheses shown in FIG.4

FIG. 6 is a posterior perspective view of the natural left and rightfacet joints between two lumbar vertebrae.

FIG. 7 is a posterior perspective view of the lumbar vertebrae shown inFIG. 6, showing one embodiment of a surgical removal of the spinousprocess and natural inferior processes and related bony structure of thesuperior vertebra and the surgical removal of the natural superiorprocesses and related bony structure of the inferior vertebra.

FIG. 8 is a posterior perspective view of the lumbar vertebrae shown inFIGS. 6 and 7, after removal of the inferior and superior halves of thenatural facet joints, illustrating the mounting of the left and rightsupport components of the cephalad prosthesis for replacing the inferiorhalves of the natural facet joints that have been removed onto fixationelements secured within the superior vertebra.

FIG. 9 is a posterior perspective view of the lumbar vertebrae shown inFIG. 8, illustrating the placement of the transverse rod and left andright cephalad arm components of the cephalad prosthesis onto thesuperior vertebra for replacing the inferior halves of the natural facetjoints that have been removed.

FIG. 10 is a posterior perspective view of the lumbar vertebrae shown inFIG. 9, illustrating the mounting of caudal prostheses for replacing thesuperior halves of the natural facet joint that have been removed ontofixation elements secured within the inferior vertebra.

FIG. 11 is a posterior perspective view of the lumbar vertebrae shown inFIG. 10, illustrating the fixation of the cephalad artificial facetjoint structures in an articulating configuration with the caudalartificial facet joint structures.

FIG. 12 is an exploded perspective view of an alternative embodiment ofcephalad and caudal prostheses for replacing, respectively, the inferiorand superior halves of a natural facet joint.

FIG. 13 is an assembled perspective view of the prostheses shown in FIG.12.

FIG. 14 is a posterior perspective view of the lumbar vertebrae shown inFIGS. 6 and 7, after removal of the inferior and superior halves of thenatural facet joints, illustrating the placement of fixation elements ofthe cephalad prosthesis within the superior vertebra.

FIG. 15 is a posterior perspective view of the lumbar vertebrae shown inFIG. 14, illustrating the mounting of the left and right cephaladsupport components of the cephalad prosthesis onto the cephalad fixationelements.

FIG. 16 is a posterior perspective view of the lumbar vertebrae shown inFIG. 15, illustrating the mounting of the left and right caudal supportcomponents of the caudal prosthesis onto the caudal fixation elementsthat have been secured with the inferior vertebra.

FIG. 17 is a posterior perspective view of the lumbar vertebrae shown inFIG. 16, illustrating the mounting of the arm components of the cephaladprosthesis onto the support components of the cephalad prosthesis.

FIG. 18 is a posterior perspective view of the lumbar vertebrae shown inFIG. 14, illustrating the mounting of the left and right cephaladsupport components of the cephalad prosthesis onto the cephalad fixationelements.

FIG. 19 is a perspective view of an alternative embodiment of thecephalad prosthesis fixation element shown in FIG. 1, illustrating thebody of the fixation element being of a stem configuration.

FIG. 20 is a perspective view of another alternative embodiment of thecaudal prosthesis fixation element shown in FIG. 1, illustrating thebody of the fixation element being of a stem configuration.

FIG. 21 is an exploded perspective view of an alternative embodiment ofcephalad and caudal prostheses for replacing, respectively, the inferiorand superior halves of a natural facet joint.

FIG. 22 is an assembled perspective view of the prostheses shown in FIG.21.

FIG. 23 is a posterior perspective view of the lumbar vertebrae shown inFIGS. 6 and 7, after removal of the inferior and superior halves of thenatural facet joints, illustrating the fixation of the cephalad andcaudal prostheses shown in FIG. 21 on the vertebrae.

FIG. 24 is an exploded perspective view of an alternative embodiment ofcephalad and caudal prostheses for replacing, respectively, the inferiorand superior halves of a natural facet joint, illustrating the arm ofthe cephalad prosthesis including a female fitting and the artificialfacet joint structure of the cephalad prosthesis including acomplementary male fitting.

FIG. 25 is an alternative embodiment of the cephalad prosthesis shown inFIG. 24, illustrating multiple attachment sites on the arm forattachment of the artificial facet joint structure.

FIG. 26 is an alternative embodiment of the cephalad prosthesis shown inFIG. 24, illustrating the arm including a male fitting and theartificial facet joint structure including a female fitting.

FIG. 27 is an alternative embodiment of the cephalad prosthesis shown inFIG. 26, and illustrating the artificial facet joint structure having afitting elongated relative to FIG. 26.

FIG. 28 is an assembled perspective view of the cephalad and caudalprostheses shown in FIG. 24.

FIG. 29 is posterior perspective view of the lumbar vertebrae shown inFIG. 8, illustrating a bore reamed in a pedicle of the inferiorvertebra.

FIG. 30 is a posterior perspective view of the lumbar vertebrae shown inFIG. 29, illustrating the placement of a caudal sleeve, with a supportmounted on the sleeve, within the bore.

FIG. 31 is a posterior perspective view of the lumbar vertebrae shown inFIG. 30, illustrating the mounting of a caudal arm, carrying anartificial facet joint structure, on the caudal sleeve.

FIG. 32 is a posterior perspective view of the lumbar vertebrae shown inFIG. 31, illustrating a bore reamed in the complementary pedicle of thesuperior vertebra.

FIG. 33 is a posterior perspective view of the lumbar vertebrae shown inFIG. 32, illustrating the placement of a cephalad sleeve, with a supportand arm mounted on the sleeve, within the bore.

FIG. 34 is a posterior perspective view of the lumbar vertebrae shown inFIG. 33, illustrating the mounting on an artificial facet jointstructure on the cephalad arm.

FIG. 35 is a posterior perspective view of the lumbar vertebrae shown inFIG. 34, illustrating the use of a pedicle screw to secure the mountingof the cephalad support and arm.

FIG. 36 is an exploded perspective view of a pair of cephalad prosthesesjoined by a transverse brace.

FIG. 37 is an exploded perspective view of a pair of caudal prosthesesjoined by a transverse brace.

FIG. 38 is a posterior perspective view of the lumbar vertebrae shown inFIG. 35, illustrating bilateral cephalad and caudal prostheses fixed onthe vertebrae using a pair of braces.

FIG. 39 is a sectional view of a bore within bone, and illustrating theplacement of mesh material and a sleeve within the bore.

The invention may be embodied in several forms without departing fromits spirit or essential characteristics. The scope of the invention isdefined in the appended claims, rather than in the specific descriptionpreceding them. All embodiments that fall within the meaning and rangeof equivalency of the claims are therefore intended to be embraced bythe claims.

DETAILED DESCRIPTION OF THE INVENTION

Although the disclosure hereof is detailed and exact to enable thoseskilled in the art to practice the invention, the physical embodimentsherein disclosed merely exemplify the invention that may be embodied inother specific structures. While the preferred embodiment has beendescribed, the details may be changed without departing from theinvention, which is defined by the claims.

I. VERTEBRAL PROSTHESES

FIGS. 4 and 5 illustrate prostheses for replacing the superior andinferior portions of natural facet joints 32 (See also FIG. 3). Theprostheses are desirably fixed to vertebral bodies 14 following thesurgical removal of the respective natural facet joint portions from thevertebral bodies 14.

The cephalad 36 prosthesis is sized and configured for replacement ofthe natural inferior facet of a facet joint 32 following removal of thenatural inferior facet of the facet joint 32. The caudal prostheses 38are sized and configured for replacement of the natural superior facetof a facet joint 32 following removal of the natural superior facet ofthe facet joint 32.

As best shown in FIG. 11, the prostheses 36 and 38 are desirably used inarticulated association between a given pair of vertebral bodies 14. AsFIG. 11 shows, the caudal and cephalad prostheses 36 and 38 form anarticulated system that permits total (superior and inferior) facetjoint replacement of one or more natural facet joints 32. The system canprovide a succession of entirely artificial facet joint structuresbetween two vertebral bodies 14 or along a length of the spinal column10.

A. The Cephalad Prosthesis

The cephalad 36 prosthesis shown in FIGS. 4 and 5 is so designatedbecause it provides one or more artificial facet joint structures 40 forrepair/replacement of the inferior facet of a natural facet joint 32.The prosthesis 36 allows for the removal of injured, diseased and/ordeteriorating natural inferior articular surfaces 28 and supporting bonystructure on the vertebra 12 above the facet joint 32. The artificialstructures 40 serve to replace the natural inferior processes 28 andsupporting bone of the vertebral body 14, which have been desirablyremoved prior to mounting the prosthesis 36 on the vertebral body 14, aswill be described in greater detail later.

The artificial facet joint structures 40 articulate with the superiorfacet of the facet joint 32. The superior facet can comprise the naturalsuperior portions of the facet joint 32 (i.e., the natural superiorarticular surfaces 26 and supporting bony structure on the vertebralbody 14 below the facet joint 32). Desirably, however, the superiorfacet comprises an artificial facet joint structure 42 formed by acaudal joint replacement prosthesis 38.

The cephalad prosthesis 36 is a modular unit comprising a pair offixation elements 44 (left and right), a pair of supports 46 (left andright), a pair of arms 48 (left and right), and a brace 50. The modularunit allows assembly of the components in situ on a vertebra. Thecephalad prosthesis 36 may be formed of a material commonly used in theprosthetic arts including, but not limited to, polyethylene, rubber,titanium, chrome cobalt, surgical steel, bony in-growth sintering,sintered glass, artificial bone, ceramics, or a combination thereof.

The left and right fixation elements 44 are fixed to the left and rightpedicles 16 respectively, in a position that desirably best assurestheir fixation to cortical and/or cancellous bone. In the illustratedembodiment, the fixation elements 44 take the form of pedicle screws ornails. The fixation elements 44 are adapted to extend into the right andleft pedicles 16 of the vertebral body and serve to anchor theprosthesis 36 in place in an orientation that best assures a secure anddurable attachment to bone.

The supports 46 each carry at least one opening 52 sized and configuredto accommodate passage of a fixation element 44 to permit mounting of asupport 46 on the fixation element 44. The supports 46 are therebyplaced on the vertebra 12 in a position dictated by the placement andorientation of the fixation elements 44.

The supports 46 also each have an opening 54 to permit passage of thebrace 50. In the illustrated embodiment, the brace 50 takes the form ofa transverse rod. Similar to the 46 supports, the left and rightcephalad arms 48 have openings 56 to permit passage of the brace 50. Thebrace 50 is sized to extend across the laminae 20 of a vertebral body 14and passes through the support openings 54 and the arm openings 56 tohold the supports 46 and arms 48 to thereby stabilize the prosthesis 36.

Each arm 48 carries an artificial facet joint structure 40 forrepairing/replacing the inferior facet of a natural facet joint 32. Theposition of the arms 48 may be adjusted along the brace 50 to bring theartificial facet joint structures 40 of the cephalad prosthesis 36 inarticulating configuration with the natural superior facet of the facetjoint 32 or an artificial facet joint structure 42 formed by a caudaljoint replacement prosthesis 38. The arms 48 can then be secured bylocking pins 58 or other suitable mechanism in a desired position.

B. The Caudal Prosthesis

The caudal prostheses 38 shown in FIGS. 4 and 5 are so designatedbecause they create artificial facet joint structures 42 for thesuperior facet of a natural facet joint 32. The caudal prostheses 38allow for the removal of injured, diseased and/or deteriorating naturalsuperior articular surfaces 26 and supporting bony structure on thevertebral body 14 below the facet joint 32. The artificial structures 42serve to replace the natural superior processes 26 and supporting boneof the vertebral body 14, which have been desirably removed prior tomounting the prosthesis 38 on the vertebral body 14. This aspect will bedescribed in greater detail later.

In use, the artificial facet joint structure 42 articulates with theinferior facet of the facet joint 32. The inferior facet can comprisethe natural inferior portions of the facet joint 32 (i.e., the naturalinferior articular surfaces 28 and supporting bony structure on thevertebral body 14 above the facet joint 32). Desirably, however, theinferior facet comprises an artificial facet joint structure 40 formedby a cephalad joint replacement prosthesis 36, as previously described.

Each prosthesis 38 comprises an artificial facet joint structure 42 anda fixation element 62. Desirably, as FIGS. 4 and 5 illustrate, a pair offixation elements 62 (right and left) and a pair of artificial facetjoint structures 42 (right and left) are provided to permit bilateralfacet joint replacement. The left and right fixation elements 62 arefixed to the left and right pedicles 16 respectively, in a position thatbest assures their fixation to cortical and/or cancellous bone. In theillustrated embodiment, the fixation elements 62 take the form ofpedicle screws or nails. The fixation elements 62 are adapted to extendinto the right and left pedicles 16 of the vertebral body 14 and serveto anchor the prostheses 38 in place in an orientation that best assuresa secure and durable attachment to bone.

Each artificial facet joint structure 42 has at least one opening 64sized and configured to accommodate passage of a fixation element 62 topermit mounting of the artificial facet joint structure 42 on a fixationelement 62. The artificial facet joint structures 42 are thereby placedon the vertebra 12 in a position dictated by the placement andorientation of the fixation elements 62.

The artificial facet joint structures 42 articulate with the naturalinferior facet portion of the facet joint 32 or an artificial facetjoint structure 40 formed by a cephalad joint replacement prosthesis 36,as previously described.

The caudal prostheses 38 may be formed of a material commonly used inthe prosthetic arts including, but not limited to, polyethylene, rubber,titanium, chrome cobalt, surgical steel, bony in-growth sintering,sintered glass, artificial bone, ceramics, or a combination thereof.

C. Artificial Facet Structure Configuration

In the prostheses 36 and 38, each artificial facet joint structure 40and 42 creates a bearing surface having a configuration that facilitatesarticulation with the bearing surface of another artificial facet jointstructure 40 or 42. The particular geometry for the bearing surfaceconfiguration for a given artificial facet joint structure 40 and 42 canvary. It can, for example, be concave, convex, or flat. It may alsoinclude a hybrid of curved and flat bearing surface designs, i.e.,Miniscal, hinge, etc.

The radii of two articulating bearing surface configurations aredesirably selected and matched, taking into account the material fromwhich the surfaces are formed, to minimize contact stress duringarticulation. The features of the two bearing surfaces (as well as thevarious other features of the facet joint structures) may also bechosen, if desired, to duplicate the natural articulation of the naturalfacet joint. Alternatively, the features of the two bearing surfaces (aswell as the various other features of the facet joint structures) can bechosen to permit the treated motion segment to experience a lesser orgreater degree of articulation than that allowed by the natural motionsegment.

For example, in the embodiment illustrated in FIG. 4, the cephaladprosthesis 36 includes artificial facet structures 40 employinggenerally convex surfaces 66, forming hemisphere-like artificial facetjoint structures. In this arrangement, the caudal prostheses 38 includeartificial facet structures 42 employing generally complementary concavesurfaces 68, forming socket-like artificial facet joint structures thatarticulate with the hemisphere-like artificial facet joint structures.It should be appreciated that the articulating surfaces 40 and 42 can bereversed, with the artificial facet structures 40 of the cephaladprosthesis 36 employing generally socket-like surfaces, and theartificial facet structures 42 of the caudal prostheses 38 employinggenerally hemisphere-like surfaces.

Alternatively, a Miniscal bearing design could be employed, utilizing aconformal curved surface as one artificial facet joint structure 40 or42, with the bearing side of the opposed artificial facet jointstructure 40 or 42 having an essentially flat surface. Ahemiarthroplasty design could also alternatively be employed, in whichone surface of the opposing surfaces does not incorporate the use of anartificial facet joint structure 40 or 42.

In another arrangement, one surface of an artificial facet jointstructure 40 or 42 can have bearing articulation on both sides of thecomponent and have opposing articulation with a receiving artificialfacet joint structure 40 or 42 having opposing mating bearing surfaces.

A variety of materials are suitable for the artificial facet jointstructures 40 and 42. Ceramic or ceramic in opposition with a chromealloy can be used. Suitable stainless steel, including 3161, or titaniumalloys, with or without the use of surface hardening and overlay, orhard surface coatings, including zirconia and alumina, can also beemployed. The metal surfaces can be made from cast, wrought, hot-forged,or powder-metal consolidated sintered materials. Any of these metals orcombination of metals and ceramics can be used in articulation with eachother. Biocompatible polymers, e.g., polyethylene, can also be used inarticulation with the metals, ceramic, and surface-hardened metals justdescribed. Ultra High Molecular Weight Polyethylene can further begamma-irradiated, as-molded or as-machined.

The radii of articulating artificial facet joint structures 40 and 42are desirably closely matched to provide contact stress values less thana given threshold value. The desired contact stress value changes withthe material employed.

For example, the contact stress value for metal-to-metal bearingcombinations is desirably less than about 25,000 psi, and preferablyless than 12,000 psi. For polymer surfaces bearing against a metal,ceramic, or surface-hardened metal counter bearing surface, the contactstress value is desirably less than 10,000 psi, and preferably less than5,000 psi.

For a given material to achieve a desired contact stress value less thanthe threshold value, the appropriate radii is desirably chosen. Thus,the desired radii may change as material changes.

D. Total Facet Replacement Using the Cephalad and Caudal Prostheses

FIG. 6 shows a normal natural human vertebral facet joint 32, e.g.,L4-L5. In some cases of disease or trauma, it may be desirable to removethe superior and inferior facets of the natural facet joint 32 andreplace them respectively with the caudal prostheses 38 and the cephaladprosthesis 36.

FIGS. 6 and 7 show the exposed spinous process 22, lamina 20, and facetjoint 32 of the L4-L5 joint. In this embodiment, a portion of theinferior lamina 20 and the inferior facet of the natural facet joint 32(e.g., the articulated inferior processes 28 and its supporting bone ofthe vertebral body 14 above the facet joint 32) are removed. The lamina20 is cut for a wide decompressive laminectomy along a decompressivesuperior-to-inferior resection line on both sides of the vertebral body14. The removed natural anatomy is replaced with the cephalad prosthesis36. The superior facet of the natural facet joint 32 (e.g., thearticulated superior process 26 and its supporting bone of the targetedvertebral body 14) is also removed. Desirably, the mamillary process,the accessory process, a portion of the transverse process, and aportion of the pedicle is removed by being rongeured or reamed. Theremoved natural anatomy is replaced with the caudal prosthesis 38. Thecephalad prosthesis 36, as described above, can be installed over thelamina 20, either before or after placement of the caudal prosthesis 38.

As best shown in reference to FIG. 7, the embodiment of a surgicalprocedure exposes the spinous process 22, lamina 20, and facet joints 32at a desired level of the spine 10 using any method common to those ofskill in the medical arts.

A portion of the spinous process 22 of the superior vertebra 12 isdesirably removed, as depicted by phantom lines in FIG. 7, using anymeans common in the field. The inferior facet of the facet joint 32 iscut at or near a selected resection line. Most of the lamina 20 isdesirably preserved, as is the facet joint capsule, which may be openedand folded back. The facet joint capsule may be cut perpendicular to itsdirection. The natural inferior facet of the facet joint 32 may then beretracted from the superior facet. Once the inferior and superior facetsof the facet joint are separated, the cut inferior bone, e.g., theinferior articular process 28 and its supporting bone, of the upper halfof the joint (e.g., the cut inferior portion of the L4 vertebra in theL4-L5 joint) may be removed, as also depicted by phantom lines in FIG.7. Alternatively, it may be possible to remove the cut inferior bonewhile simultaneously separating the facet joint 32.

Prominent bone of the superior facet of the natural facet joint 32,e.g., the superior articular process 26 and its supporting bone, may bealso removed, as also depicted by phantom lines in FIG. 7, using anymeans common in the field. The superior facet of the natural facet joint32 may also be trimmed to decompress the adjacent nerve root. A reameror any other instrument that is useful for grinding or scraping bone,may be used to ream the superior facet of the facet joint 32 into thepedicle 16, to reach the geometry shown in FIG. 8, which is desirablysuitable for receiving the caudal prosthesis 38.

With reference to FIG. 8, a cephalad support 46 is mounted on each ofthe cephalad fixation elements 44 and the fixation elements 44 are thenplaced in a desired position on the pedicles 16 (with one fixationelement 44 on each of the right and left pedicles 16) and screwedsecurely into the superior vertebral body 14.

As FIG. 9 illustrates, the cephalad arms 48 are then placed medial tothe left and right supports 46. The brace 50 is then passed through theopenings 54 and 56 of the supports 46 and arms 48.

The caudal artificial facet joint structures 42 are then mounted on thecaudal fixation elements 62 and the fixation elements 62 are then placedin a desired position on the pedicles 16 (with one fixation element 62on each of the right and left pedicles 16) and screwed securely into theinferior vertebral body, as shown in FIG. 10.

With reference to FIG. 11, the cephalad arms 48 are then positioned tobring the artificial facet joint structures 40 of the cephaladprosthesis 36 in articulating configuration with the artificial facetjoint structures 42 of the caudal prosthesis 38. The arms 48 are thensecured in the desired position by use of locking screws 58 or othersuitable mechanism.

Further details of surgical procedures suitable for installing theprostheses are described in co-pending U.S. patent application Ser. No.09/693,272, filed Oct. 20, 2000, and entitled “Facet ArthroplastyDevices and Methods,” which is incorporated herein by reference.

II. FIRST ALTERNATIVE EMBODIMENT

1. Cephalad Prosthesis

FIGS. 12 and 13 show alternative embodiments of a cephalad 70 and acaudal 72 vertebral prosthesis. Similar to the previous embodiment, thecephalad prosthesis 70 is a modular unit comprising left and rightfixation elements 74, left and right supports 76, left and right arms78, left and right artificial facet joint structures 79 for the superiorfacet of a natural facet joint 32, and a transverse brace 80 that allowsassembly of the components in situ. Components are mounted in situ onthe fixation elements 74 that are secured to the pedicle 16 in anorientation that provides secure fixation to bone.

The left and right fixation elements 74 are fixed to the left and rightpedicles 16 respectively, in a position that best assures their fixationto cortical and/or cancellous bone. In the illustrated embodiment, thefixation elements 74 take the form of pedicle screws or nails. Thefixation elements 74 are adapted to extend into the right and leftpedicles 16 of the vertebral body 14 and serve to anchor the prosthesis70 in place in an orientation that best assures a secure and durableattachment to bone.

The fixation elements 74 have a threaded body 82 configured to screwinto the pedicle 16. A spacing collar 84 may be provided to addadditional length to the fixation element 74 if necessary to assure itsfixation in the vertebra 12. A nut 86 may be provided to couple with awrench or other tool to facilitate screwing the fixation element 74 intothe vertebra 12. An end portion 88 passes through an opening 90 in thesupport 76 and permits attachment of the support 76 to be secured by nut92 or other fixation means, e.g., by threaded engagement.

FIG. 19 shows an alternative embodiment of a fixation element 74 inwhich the body 82 is a stem configuration. In this arrangement, thefixation element 74 is placed into a hole that has been reamed into thebone and secured by adhesive or boney in-growth material. The stem mayinclude a series of serrated vanes 94 to prevent rotation in bone.

With reference back to FIG. 12, in the illustrated embodiment, thesupport 76 takes the form of a right angle connector. The support 76 issized and configured to couple with the arm 78, e.g., by Morse taper.The arm 78 is a generally cylindrical member having a first bore 96configured, e.g., tapered, to receive the support 76. A second bore 98permits the arm 78 to couple with an artificial facet joint structure 78for repairing/replacing the inferior facet of a natural facet joint 32,e.g., the bore 98 may be tapered to couple with the artificial facetjoint structure 78 by Morse taper. A third bore 100 receives a fixationelement 102, e.g., screw, to secure the arm 78 to brace 80.

The transverse brace 80 comprises a right component 104 and a leftcomponent 106. An end opening 108 in each of the components 104 and 106receives a fixation element 102 to fix the right and left components 104and 106 to the right and left arms 78 respectively, e.g., by threadedengagement. Each component 104 and 106 desirably has a medial opening110. The medial openings 110 are sized and configured to overlap andpermit passage of a fixation element 112, which may be secured by nut114 or similar mechanism, to thereby couple the components 104 and 106together to form the transverse brace 80. Similar to brace 50 describedin relation to the previous embodiment, the brace 80 extends across thelaminae 20 of a vertebral body 14, providing a width-adjustableload-bearing support that further stabilizes the prosthesis 70.

2. Caudal Prosthesis

With continued reference to FIGS. 12 and 13, the caudal prosthesis 72 isa modular unit comprising left and right fixation elements 118, left andright artificial facet joint structures 120, and a transverse brace 122that allows assembly of the components in situ. Components are mountedin situ on the fixation elements 118 that are secured to the pedicle 16in an orientation that provides secure fixation to bone.

The left and right fixation elements 118 are fixed to the left and rightpedicles 16 respectively, in a position that best assures their fixationto cortical and/or cancellous bone. In the illustrated embodiment, thefixation elements 118 take the form of pedicle screws or nails. Thefixation elements 118 are adapted to extend into the right and leftpedicles 16 of the vertebral body 14 and serve to anchor the prosthesis72 in place in an orientation that best assures a secure and durableattachment to bone.

The fixation elements 118 have a threaded body 124 configured to screwinto the pedicle 16. A nut 126 may be provided to couple with a wrenchor other tool to facilitate screwing the fixation element 118 into thevertebra 12. An end portion 128 is configured to couple with a support120, e.g., may be tapered to couple with the support 120 by Morse taper.

FIG. 20 shows an alternative embodiment of a fixation element 118 inwhich the body 124 is a stem configuration. In this arrangement, thefixation element 118 is placed into a hole that has been reamed into thebone and secured by adhesive or boney in-growth material. The stem mayinclude a series of serrated vanes 94 to prevent rotation in bone.

Turning back to FIGS. 12 and 13, each artificial facet joint structure120 is sized and configured to be mounted on a fixation element 118,e.g, has a tapered bore 130 to mate with tapered end portion 128 of thefixation element 118. A second bore 132 receives a fixation element 136to secure the artificial facet joint structure 120 to the caudal brace122, e.g., by threaded engagement. The artificial facet joint structures120 articulate with the natural inferior facet portion of the facetjoint 32 or an artificial facet joint structure 79 formed by a cephaladjoint replacement prosthesis 70, as previously described.

In the illustrated embodiment, the brace 122 takes the form of atransverse bar. The brace 122 desirably has right and left end openings134 that receive fixation elements 136 for attachment to the right andleft supports 122 respectively, e.g., by threaded engagement. The brace122 extends across the laminae 20 of the inferior vertebra 12 to providea width-adjustable load-bearing support to further stabilize the caudalprosthesis 72.

3. Total Facet Replacement Using the Cephalad and Caudal Prostheses

In a surgical procedure for total facet replacement using the cephaladand caudal prostheses 70 and 72, the spinous process 22 along with theinferior articular process 28 and its supporting bone, of the upper halfof the joint 32 (e.g., the cut inferior facet of the L4 vertebra in theL4-L5 joint) may be removed, as previously described (see FIG. 7).Prominent bone of the superior facet of the natural facet joint 32,e.g., the superior articular process 26 and its supporting bone, may bealso removed, as also previously described, using any means common inthe field (see FIG. 7).

As shown in FIG. 14, a cephalad fixation element 74 is placed in adesired position on each of the right and left pedicles 16 of thesuperior vertebra 12 and secured in the vertebral body 14 by screwingthe nut 86.

A cephalad support 76 is then mounted on each of the fixation elements74 and secured with a nut 92 or other suitable means, as seen in FIG.15. With reference to FIG. 16, a caudal fixation element 118 is thenplaced in a desired position on each of the right and left pedicles 16of the inferior vertebra 12 and secured in the vertebral body 14 byscrewing the nut 126. A caudal support 120 is then mounted on each ofthe fixation elements 118, as FIG. 16 also shows.

Referring now to FIG. 17, a cephalad support 76 and a cephalad arm 78are then mounted on each of the cephalad fixation elements 74. Theartificial facet joint structures 79 of cephalad prosthesis 70 are thenbrought into articulating configuration with artificial facet jointstructures 121 of the caudal prosthesis 72 and the arms 78 are securedby nuts 92 or other suitable mechanism.

As seen in FIG. 18, the right cephalad brace component 104 is then fixedto the to right cephalad arm 78 and the left cephalad brace component106 is fixed to left cephalad arm 78 with fixation elements 102 or othersuitable mechanism. The left and right brace components 104 and 106 arethen secured to each other with a fixation element 112 or other suitablemeans to form a unitary cephalad transverse brace 80. The caudalsupports 120 can then be coupled with the caudal brace 122 usingfixation elements 136 or other suitable means.

III. SECOND ALTERNATIVE EMBODIMENT

FIGS. 21-23 illustrate a cephalad prosthesis 138 and a caudal prosthesis140 similar to the embodiments shown in FIGS. 12 and 13, and like partswill be given like reference numerals.

Each arm 78 of the cephalad prosthesis 138 includes a slot 142 forreceiving a brace 144. In the illustrated embodiment, the brace 144takes the form of a curvilinear transverse rod. The brace 144 is securedto the arms 78 by fixation elements 146.

Similar to the cephalad prosthesis 138, each support 120 of the caudalprosthesis 140 has a slot 148 for receiving a brace 150. In theillustrated embodiment, the brace 150 takes the form of a curvilineartransverse rod. The brace 150 is secured to the supports 120 by fixationelements 152.

The prostheses 138 and 140 are secured in the vertebrae by surgicalprocedure, as previously described (see also FIGS. 14-18).

IV. THIRD ALTERNATIVE EMBODIMENT

FIGS. 24-28 illustrate another embodiment of a cephalad prosthesis 154and a caudal prosthesis 156.

1. Cephalad Prosthesis

Similar to the previous embodiments, the cephalad prosthesis 154 is amodular unit comprising a fixation element 158, a support 160, and anarm 162 carrying an artificial facet joint structure 176 that allowassembly of the components in situ. A pair of fixation elements 158(right and left) are desirably provided and sized and configured to beare secured to the right and left pedicles 16 in an orientation thatprovides secure fixation to bone. Components are mounted in situ on thefixation elements 158 that are secured to the pedicle 16 in anorientation that provides secure fixation to bone.

In the illustrated embodiment, each fixation element 158 takes the formof a sleeve 164 and a pedicle screw 166. The sleeve 164 is sized andconfigured for insertion into a bore 168 that has been reamed into thepedicle 16 (see also FIG. 28). The sleeve 164 provides an increasedsurface area of attachment, further securing the attachment of theprosthesis 154 to bone. Desirably, the sleeve 164 includes a pluralityof vanes 170 that resist rotation of the sleeve 164 in bone to furthersecure the sleeve 164 within the vertebra 12. In one alternativeembodiment, the sleeve 164 can comprise an expandable sleeve whichexpands (in a manner similar to a wall anchor) in diameter (desirablywithin the bore 168) when the screw 166 is advanced through the sleeve164.

The sleeve 164 can be secured for long-term fixation within the bore 168by adhesive, e.g., bone cement. Alternatively, the sleeve 164 couldincorporate a boney in-growth outer surface to which the surroundingbone could grow and adhere. Desirably, the sleeve 164 would fit tightlywithin the bore 168, with the distal section of the screw 166 anchoredwithin the cancellous bone, thereby securely anchoring the sleevemechanically while allowing the surrounding bone to biologically adhereto the outer surface of the sleeve. As another alternative (as shown inFIG. 39), the sleeve 164 can be secured by boney in-growth with a meshmaterial 172 placed within the bore 168. The sleeve 164 is then placedwithin the bore 168, such that it is surrounded by the mesh material172. The mesh material 172 can be made of titanium, chrome, steel, orother suitable metal alloy for boney in-growth to infiltrate. Thepedicle screw 166 provides interim mechanical fixation until the sleeve164 joins the bone.

A support 160 and arm 162 are integrally formed with the sleeve 164 orotherwise securely mounted on the sleeve 164. An opening 174 in thesleeve 164 extends through the support 160 and serves to receive thepedicle screw 166, e.g., by screwing the pedicle screw 166 into thesleeve 164. The arm 162 is sized and configured to couple with anartificial facet joint structure 176, e.g., by Morse taper or othersuitable mechanism that permits rotation (if desired) of the artificialfacet joint structure 176 with respect to the support 160 to enableproper orientation of the artificial facet joint structure 176 with thecaudal prosthesis 156. In the arrangement illustrated in FIG. 24, theartificial facet joint structure 176 includes a male fitting 178 and thearm 162 includes a complementary female fitting 180. Alternatively, themale fitting 178 and female fitting 180 could comprise a non-circularfitting arrangement (oval, slotted, triangular, polygonal, etc.) thatreduces or prevents relative motion between the fittings 178 and 180,but allows the facet joint structure 176 to assume one of two (or more)predetermined positions.

The arm 162 can be of a fixed length. In a representative embodiment,the arm 162 is approximately 1 cm in length. As FIG. 25 shows, toaccommodate individual variations in anatomy and customize theprosthesis 154 to a given individual, the arm 162 may include more thanone attachment site 182 for the artificial facet joint structure 176 atdifferent distances along the arm 162. It is contemplated that thenumber and position of the attachment sites 182 may vary.

FIG. 26 illustrates an alternative embodiment, in which the arm 162includes a male fitting 178 and the artificial facet joint structure 176includes a complementary female fitting 180. FIG. 27 illustrates how thefitting 178 may be extended to accommodate individual variations inanatomy. For example, the fitting 178 may be provided in a series ofdifferent lengths, e.g., 5, 6, 7, 8, 9 or 10 mm.

2. Caudal Prosthesis

Also similar to the previous embodiments, the caudal prosthesis 156 is amodular unit comprising a fixation element 184, a support 186, and anartificial facet joint structure 200 that allow assembly of thecomponents in situ. A pair of fixation elements 184 (right and left) aredesirably provided and sized and configured to be secured to the rightand left pedicles 16 in an orientation that provides secure fixation tobone. Components are mounted in situ on the fixation elements 184 thatare secured to the pedicle 16 in an orientation that provides securefixation to bone.

In the illustrated embodiment, each fixation element 184 takes the formof a sleeve 190 and a pedicle screw 192, similar to the cephaladprosthesis 154. The sleeve 190 is sized and configured for placementwithin a bore 168 reamed in bone and can be secured by adhesive or byboney in-growth, as previously described. The sleeve 190 provides anincreased surface area of attachment, further securing the attachment ofthe prosthesis 156 to bone. Desirably, the sleeve 190 includes aplurality of vanes 170 that resist rotation of the sleeve 190 in bone tofurther secure the sleeve 190 within the vertebra 12, as also previouslydescribed.

A support 186 is integrally formed with the sleeve 190 or otherwisesecurely mounted on the sleeve 190. An opening 194 in the sleeve 190extends through the support 186 and serves to receive the pedicle screw192, e.g., by screwing the pedicle screw 192 into the sleeve 190. Thesupport 186 is adapted to couple with the artificial facet jointstructure 200. For example, in the illustrated embodiment, the support186 carries a lip 196 which mates with a complementary lip 198 on thestructure 200 to couple the structure with the support 186. Theartificial facet joint structure 200 is sized and configured toarticulate with the natural inferior facet portion of the facet joint 32or an artificial facet joint structure 176 carried by the cephaladprosthesis 154 (FIG. 28).

3. Total Facet Replacement Using the Cephalad and Caudal Prostheses

In a surgical procedure for total facet replacement using the cephaladand caudal prostheses 154 and 156, some or all of the spinous process22, along with the inferior articular process 28 and its supportingbone, of the upper half of the joint 32 (e.g., the cut inferior facet ofthe L4 vertebra in the L4-L5 joint) may be removed, as previouslydescribed (see FIG. 7). Prominent bone of the superior facet of thenatural facet joint 32 (e.g., the superior articular process 26 and itssupporting bone), may be also removed, as also previously described,using any means common in the field (see FIG. 7).

As shown in FIG. 29, a bore 168 is reamed in a desired position in thepedicle 16 of the inferior vertebra 12 by conventional methods. Ifdesired, mesh material 172 promoting boney in-growth is placed withinthe bore 168, as previously described (see FIG. 39). Alternatively, anadhesive material is placed within the bore 168 or along the outside ofthe caudal sleeve 190, as also previously described. The sleeve 190,with attached support 186, is then placed within the bore 168 andsecured with a pedicle screw 192, as seen in FIG. 30.

The caudal artificial facet joint structure 200, is then mounted on thecaudal support 186, as shown in FIG. 31.

As FIG. 32 illustrates, a bore 168 is reamed in a desired position inthe corresponding pedicle 16 of the superior vertebra 12. The cephaladsleeve 164, with attached support 160 and arm 162 components, is placedwithin the bore 168 and secured by adhesive and/or boney in-growth meshmaterial 172, as shown in FIG. 39.

Next, with reference to FIG. 34, the cephalad artificial facet jointstructure 176 is mounted on the cephalad arm 162. The position of theartificial facet structure 176 is then adjusted to assure articulationbetween artificial facet joint structures 200 and 176. The arm 162 isthen secured in the desired position with a pedicle screw 166, as shownin FIG. 35.

While FIGS. 29-35 illustrate unilateral facet joint replacement, it isoften desirable to perform a bilateral facet joint replacement. In suchan arrangement, as shown in FIG. 36, the left and right cephaladprostheses 154 may be coupled with a transverse brace 202. It isapparent that the brace 202 can be variously constructed. In theillustrated embodiment, brace 202 includes a right component 204 and aleft component 206. An end opening 208 in each of the components 204 and206 receives a fixation element 210 to fix the right and left components204 and 206 to the right and left arms 162 respectively, e.g., bythreaded engagement. Each component 204 and 206 desirably also has amedial opening 212. The medial openings 212 are sized and configured tooverlap and permit passage of a fixation element 214, which may besecured by a nut 216 and bolt 214 or similar mechanism, to therebycouple the components 204 and 206 together to form the transverse brace202. The brace 202 extends across the laminae 20 of a vertebral body 14,providing load-bearing support that further stabilizes the prostheses154.

Similarly, as seen in FIG. 37, the left and right caudal prostheses 156may be coupled with a transverse brace 218. In the illustratedembodiment, the brace 218 takes the form of a transverse bar. The brace218 includes a pair of end openings 220 (left and right) that permitpassage of fixation elements 222 to secure the brace 218 to the left andright artificial facet joint structures 200, e.g., by threadedengagement. The braces 202 and 218 provide increased stability to theprostheses 154 and 156, as previously described.

In this arrangement, as shown in FIG. 38, left and right cephaladprostheses 154 are secured within the left and right pedicles 16,respectively, of the superior vertebra 12, as previously described. Leftand right caudal prostheses 156 are secured within the left and rightpedicles 16, respectively, of the inferior vertebra 12, as alsopreviously described. The cephalad prosthesis 154 are then coupled withbrace 202, and the caudal prostheses 156 are coupled with brace 218.

It should be understood that, while the embodiments disclosed hereingenerally describe the complete repair/replacement of a pair of naturalfacet joints, the teachings of the present invention could be equallyapplicable to the repair/replacement of a single facet joint, or eventhe repair/replacement of a single cephalad or caudal portion of asingle facet joint, or any combination thereof.

The above described embodiments of this invention are merely descriptiveof its principles and are not to be limited. The scope of this inventioninstead shall be determined from the scope of the following claims,including their equivalents.

1. A spinal stabilization device, comprising: a central spacer with atleast two arms extending therefrom and adapted to couple to adjacentvertebrae, the central spacer and the at least two arms having a unitaryconfiguration and being formed from an elastomeric material.
 2. Thespinal stabilization device of claim 1, wherein the at least two armscomprises first and second superior arms extending from a superiorportion of the central spacer and adapted to mate to a superiorvertebra, and first and second inferior arms extending from an inferiorportion of the central spacer and adapted to mate to an inferiorvertebra.
 3. A spinal stabilization device, comprising: a central spaceradapted to be positioned between posterior elements of adjacentvertebrae and adapted to limit extension of the adjacent vertebrae, andat least one pair of arms extending from opposed sides of the centralspacer and adapted to couple to a vertebra, at least a portion of the atleast one pair of arms being pliable for providing resistance tomovement of adjacent vertebrae coupled thereto.
 4. The spinalstabilization device of claim 3, wherein the at least one pair of armsand the central spacer are of a unitary construction.
 5. The spinalstabilization device of claim 4, wherein the at least one pair of armsand the central spacer are formed from an elastomeric material.
 6. Thespinal stabilization device of claim 3, wherein the at least one pair ofarms is removably coupled to the central spacer.
 7. The spinalstabilization device of claim 3, wherein at least one of the at leastone pair of arms is slidably disposed through the central spacer.
 8. Thespinal stabilization device of claim 3, wherein the central spacer has ashape selected from the group consisting of wedge-shaped, round, oval,elliptical, and rectangular.
 9. The spinal stabilization device of claim3, wherein the at least one pair of arms comprises first and second armsextending from opposed sides of the central spacer, a third armremovably coupled to the first arm, and a fourth arm removably coupledto the second arm.
 10. The spinal stabilization device of claim 3,wherein the central spacer is formed from a compressible material. 11.The spinal stabilization device of claim 3, wherein the at least onepair of arms is adjustably matable to the central spacer.
 12. The spinalstabilization device of claim 3, wherein the central spacer is adaptedto at least partially rotate relative to the at least one pair of arms.13. The spinal stabilization device of claim 3, wherein the at least onepair of arms comprises a first pair of arms extending from a firstlateral side of the central spacer, and a second pair arms extendingfrom a second, opposed lateral side of the central spacer.
 14. Thespinal stabilization device of claim 13, wherein the first pair of armsare integrally formed with one another, and the second pair of arms areintegrally formed with one another.
 15. The spinal stabilization deviceof claim 14, wherein the first and second pair of arms are adapted tomate to the central spacer at a variety of angular orientations.
 16. Thespinal stabilization device of claim 14, wherein the central spacerincludes a cross-connector extending therethrough and adapted to engagethe first and second pair of arms.
 17. The spinal stabilization deviceof claim 16, wherein the cross-connector is slidably adjustable relativeto the first and second pair of arms to allow the central spacer to bemated to the first and second pair of arms at a plurality of positions.18. The spinal stabilization device of claim 3, wherein the device issubstantially X-shaped.
 19. A spinal stabilization device, comprising: acentral spacer having first and second opposed lateral sides, thecentral spacer being adapted to be positioned between posterior elementsof adjacent superior and inferior vertebrae and adapted to limitextension of the adjacent superior and inferior vertebrae; a first pairof arms extending from the first lateral side of the central spacer, thefirst pair of arms being adapted to couple to adjacent superior andinferior vertebrae; and a second pair of arms extending from the secondlateral side of the central spacer, the second pair of arms beingadapted to couple to adjacent superior and inferior vertebrae; whereinat least a portion of at least one of the first and second pair of armsis pliable for providing resistance to movement of adjacent superior andinferior vertebrae coupled thereto.
 20. The spinal stabilization deviceof claim 19, wherein the first pair of arms are coupled to one anotherand the second pair of arms are coupled to one another, and wherein thefirst and second pair of arms are removably mated to the central spacer.21. The spinal stabilization device of claim 19, wherein the first pairof arms comprises a first arm and a second arm, and the second pair ofarms comprises a third arm and a fourth arm, and wherein the first,second, third, and fourth arms are independently removably mated to thecentral spacer.
 22. The spinal stabilization device of claim 21, whereinthe central spacer includes a first opening formed in the first lateralside and adapted to removably receive the first and second arms, and asecond opening formed in the second, opposed lateral side and adapted toremovably receive the third and fourth arms.
 23. The spinalstabilization device of claim 22, wherein the central spacer includesfirst and second locking mechanisms adapted to be disposed through thefirst and second openings for locking the first, second, third, andfourth arms to the central spacer.